Body panels of automobiles, for example, have been primarily made from cold-rolled steel sheets until now. However, in accordance with the requirements for the weight reduction of automobile bodies, the use of aluminum alloy sheets of Al—Mg base, Al—Mg—Si base, and the like has been studied recently.
Generally known methods for manufacturing these aluminum alloy sheets includes a method in which a slab is cast by a DC casting method (semi-continuous casting), the slab is subjected to scalping and the resulting slab is inserted into a batch type furnace and is subjected to a homogenization treatment (soaking) for a few hours to about ten hours, followed by a hot rolling step, a cold rolling step, and an annealing step, so that a sheet having a predetermined thickness is completed (refer to, for example, JPP3155678).
Furthermore, a twin belt casting method is known in which a pair of parallel-opposed rotating endless belts are disposed, a melt of aluminum alloy is introduced into the gap between these endless belts, and is continuously taken out while being cooled, followed by being rewound around a coil (refer to, for example, PCT WO 2002/011922 (JP2004-505774A)).
However, with respect to the above-described DC casting method, since the cooling rate of the melt during casting is a relatively low one to about ten degrees centigrade per second, intermetallic compounds, e.g., Al—(Fe•Mn)—Si, crystallized in the matrix may grow to have size of ten to several tens of micrometers, particularly in the central portion of the slab. Such a intermetallic compound may adversely affect the press formability of a final annealed sheet prepared through a rolling and annealing step.
That is, when the final annealed sheet is deformed, if the size of the intermetallic compounds is relatively large, peeling (so-called void) tends to occur between the intermetallic compound and the matrix. Consequently, microcracks starting from this peeled portion may occur, so that the press formability may be deteriorated. Furthermore, dislocations accumulate around the intermetallic compound during cold rolling, and these dislocations serve nucleation sites for recrystallization during annealing. Therefore, if the intermetallic compounds become large, the number of intermetallic compounds per unit volume is decreased and, thereby, the concentration of nucleation sites for recrystallization grains is decreased. Consequently, the recrystallized grain size increases several tens of micrometers, and the press formability is deteriorated.
In the known method, a high Mg alloy is adopted to improve the press formability. However, if the content of Mg is increased, β phases precipitates in the shape of a film at grain boundaries as time goes by after the press forming is performed and, thereby, the stress corrosion cracking resistance is deteriorated.
In the known method, steps, e.g., scalping of the slab surface after the DC casting, a homogenization treatment, hot rolling, cold rolling, and intermediate annealing, are complicated and, therefore, the cost is increased.
On the other hand, in the belt casting method, the slab prepared by continuous casting of a melt is subjected to cold rolling and, therefore, there are advantages in that the steps are simplified compared with those in the DC casting method, and the manufacturing cost can be reduced.
However, in this belt casting method as well, no study has been conducted with respect to the improvement of quality, e.g., the press formability and the stress corrosion cracking resistance of the final annealed sheet.